An abscess on a rat can start from something as minor as a scratch or bite. It causes a deep pocket of infection under the skin. The abscess will continue to grow until the pressure is released and the infection is stopped. Here is a good article on abscesses from the Rat and Mouse Club of America:

http://www.rmca.org/Articles/abcess.htm

We have found that abscesses can be "contagious" around the rattery. Abscesses are thought to be caused by bacteria and it can spread. This is NOT A PROVEN fact. It just seems peculiar that once one rat gets an abscess you notice three or four. When we encounter an abscess there are several things that we do to help stop the infection:

1. If it is anything more than a simple abscess, we consult a veterinarian

2. Use 2 Q-Tips and gently apply pressure to either side and allow the infection to come out

3. Do not squeeze the abscess as you can cause the puss and infection to be pushed into the body and not out

4. This allows the infection to escape the body. Believe it or not, Abscesses heal from the inside out unlike normal scratches and abrasions, and by leaving the infection in there it cannot heal

6. Apply anti-bacterial ointment

7. Remove the scab and apply pressure daily until the abscess is the size of a pencil eraser, then it should continue healing on it's own. The scab must be removed to allow the infection out or it will continue to cause more infection. Gently lift the scab with tweezers and remove. If you are applying anti-bacterial ointment daily the scab should stay soft and be easily lifted.

8. USE HAND SANITIZER- abscess are believed to be caused by bacteria and you will pass it on to other ratties even ratties who already have their own abscess

WHEN IN DOUBT, CONTACT YOUR VET